A closer look at the Ants of Costa Rica

This topic submitted by Mark Dalman ( Pre8lude@hotmail.com) at 9:36 PM on 5/18/05.

Jen and Michele explore Lighthouse Cave in the Bahamas

Tropical Field Courses -Western Program-Miami University

Ants have been living on the earth for more than 100 million years and can be found almost anywhere on the planet. Biologists classify ants as a special group of wasps. It is estimated that there are about 20,000 different species of ants. For this reason ants have been called Earth’s most successful species. Ants form a single family, the Formicidae, in the insect order Hymenoptera (Ants, bees, and wasps). 84 genera of ants are known from Costa Rica. Ants, like all insects, have six legs with three joints on each leg. The joints are extremely strong so they can run very quickly. If a man could run as fast as an ant (proportionally), he could run as fast as a racehorse. Ants can also lift twenty times their own body weight. Perhaps the most interesting thing is that an ant has about 250,000 brain cells. A human brain has 10,000 million, so a colony of 40,000 ants has collectively the same size brain as a human. It is this simple example of intelligence that evokes my idea that ants are smart and have calculated behavior and for them to establish a mutualistic relationship with a plant (or other organism) is very smart on their part. Along with how smart they are there are a couple of different methods to capturing these ants as well. I find ants to be very intriguing because so many of their behaviors and reasoning mimic human beings or perhaps maybe we are mimicking their behaviors?!
Ant behaviors are quite similar to human behaviors. Some examples are: ants nurturing behavior is very intensive and similar to our child day care, ant education could be closely related to a sixth grade education of learning the tricks to the trade, ants keep a distinct and constant ambient temperature of 77 deg. F for developing ants, ants do incur career specialization change if there is a loss of ants in a certain department, ants’ civic duty is second to none when it comes to large projects, ants raise a constant army of ants armed with armor, poison/stingers, and destructive jaws, ants keep a constant ratio of workers, soldiers, and reproductive ants in their social planning, ants have superior engineering skills usually digging from two different directions and meeting exactly half way, ants also evoke an understanding of nature by producing water traps and water sheds as to not ruin their home and community, and finally there is a limited free will with inter relationships more symbiotic then coercive. It is these examples of ant behavior that humbles me as a human to think we are the only organism capable of intricate thought/ behavior. However, the make up of ants is highly different then mammals in several ways.
Ants have three main parts: the head, the trunk (middle section), and the rear (metasoma). The head contains the jaws, antennae, and the compound eyes. Ants use their antennae not only for touch, but also for their sense of smell and communication. The metasoma contains the stomach and rectum. Many ants have poison sacks and/or stingers in the end of the metasoma for defense against their many predators. Ants do not have lungs. Instead, oxygen enters through tiny holes all over the body and carbon dioxide leaves through the same holes. There are no blood vessels. The heart is a long tube that pumps colorless blood form the head back to the rear and then back up to the head again. The ability to identify different ants from one another begins with an indication of whether the habitus is uniform or not. The habitus is the general appearance of the ant. Next, the important generic-level characters include: 1) mandible shape and dentition, 2) the number of antennal segments, presence of antennal club, 3) the number of segments in the club, the presence of antennal groves, 4) shape of the petiole, 5) degree of development and shape of the postpetiole. Finally pertaining mostly to our study, the Costa Rican ant genera are distributed in 14 subfamilies. Being able to place an ant in a subfamily or tribe helps in narrowing the search for a specific genera one is looking for.
Ants are social insects, which mean they live in large colonies or groups. Some colonies consist of millions of ants all going through different stages of maturation such as: egg, larva, pupa, and adult. There are three types of ants in each species, the queen, the sterile female workers, and the males. The male ants, in their short life, serve only one purpose, to mate with future queen ants. The average life expectancy of an ant is 45-60 days, so the ability to make the most out of their life is highly prevalent. The queen ant grows to adulthood, mates, and then spends the rest of her life laying eggs. A colony may have only one queen, or there may be many queens depending on the species. The job of the queen is to lay eggs which the worker ants look after. Worker ants are sterile and their tasks involve looking for food, babysitting young, and defending the nest. In this hierarchy of ants there is also a food priority shift as well. Adult ants have a shift in their feeding habits that begin with eating solid food and then transitioning into swallowing the “juice,” which they squeeze from the solid food, and discard the dry part that is left. An interesting topic to research would be to determine if the dry food material is utilized in hardening the exoskeleton of the ant or vice versa. The interconnected web of social interaction is all within a housing unit that is as different from one species to the next.
Ants build many different types of homes. Each colony of ants also has its own smell. In this way, intruders can be recognized immediately. Many ants build simple little mounds out of dirt or sand. Other ants use small sticks mixed with dirt and sand to make a stronger mound that offers protection from rain. Western harvester ants make a small mound on top, but then tunnel up to 15 feet straight down to hibernate during winter. Ant mounds consist of many chambers connected by tunnels. Different chambers are used for nurseries, food storage, and resting places for the worker ants. Some ants live in wood like termites. Army ants don’t make a home at all but travel in large groups searching for food. A special group of ants such as the Azteca genus ants form homes in Cecropia trees. It is this interaction with other species that is the most attractive quality about ants.
Many insects form interactive relationships with each other along with their environment. Some organisms develop more “specialized” and specific associations such as commensalism, parasitism, and mutualism. It is this mutualism or possibly commensalism relationship that is present between the Azteca genus of ants and Cecropia genus of trees (Purves et al. 1995). The theory is that Azteca ants are obligately associated with the Cecropia tree in a mutualistic manner (Agrawl and Dubin- Thaylor, 1999).
Cecropia trees are common pioneer colonizing trees in the neotropics (Kircher, 1997). They are the first to cultivate the soil in damaged or clear-cutted areas as well as are integral parts of second growth forests. Illustratively, Cecropia trees have thin and hollow trunks with bamboo- like rings, wide-palmed leaves, and have little branching (Kircher, 1997). The Azteca ants are an aggressive genus that is often found living within the trees.
The process of colonization of the Cecropia tree begins with conjugation. The newly mated queens then search out Cecropia saplings to initiate colonies. They chew a hole in the prostoma and enter, and plug the hole. In a short time the hole reseals, sealing the queen inside. The queen rears its first string of workers without leaving the internode. One of the most fascinating things prior to the establishment of a colony large enough to occupy all internodes, Cecropia saplings are stacks of semi, self-sufficient nest sites for ants. Saplings frequently contain numerous incipient colonies in separate internodes (Longino 1989). Mature colonies may be found inhabiting lower internodes. The uppermost internodes typically contain live queens of the local species of Azteca that are obligate inhabitants of Cecropia. Internodes below those housing live queens routinely contain dead Azteca queens, evidence of repeated colony failures. These failures occur inside sealed internodes, and are not due to interactions with other colonies. During sapling growth, colonies begin to thrive, producing workers that chew back out through the prostoma and begin gathering muellerian bodies from trichilia. Finally, there is a reduction in the number of colonies, such that one Azteca colony comes to dominate the sapling (Choe and Perlman, 1997). Workers restrict their movement to the host tree itself; they do not forage off the host tree. Colonies appear to derive all sustenance from the muellerian bodies.
The idea of a mutualistic relationship is based upon the ants are believed to derive food and shelter from the trees and the trees in return receive possible protection from herbivory, and encroaching vines, in addition to receiving nitrogen from the ant carcasses and by-products (Agrawal and Dubin- Thaler, 1999). Previous studies have found Azteca ants chewing on nearby vines and trees that contacted the host Cecropia, actively serving as a bodyguard for the tree (Janzen, 1969). Furthermore, it is believed that the Cecropia receive up to 93% of their nitrogen from the ants in the form of ant carcasses (Sagers et al., 2000). The ants, in return, receive shelter and between 19% and 47% of their food from glycogen rich mullerian bodies located on the petiole of the Cecropia leaf (Sager et al., 2000). However, data so far is incomplete when it comes to whether the ants protect the tree from herbivory. So far studies have shown that the number of ants doubled when a tree leaf incurred damage leading to the theory that ants may protect the tree from being eaten (Agrawal and Dubin- Thaler, 1999). However, the ability to know whether the ants provide a sufficient defense in combating the tree from being eaten is unknown.
To capture these ants in their habitat many mass-collecting methods have been used in biodiversity surveys of terrestrial arthropods, including Malaise traps( tent- like structures that funnel flying insects into a collecting jar), light traps, pitfall traps, sweep nets, and canopy fogging with insecticides. Although samples should ideally be representative of the community or taxon selected for investigation, capture depends on such factors as individuals’ behavior, activity level, and size. Each method has inherent sampling biases and caution must be exercised in comparing subsets of specimens across samples. Such traps as pitfall and Winkler litter sampling are used with ants most prevalently. Pitfall traps are often used for ant collecting while Winkler litter sampling has until now had rather limited use. Pitfalls are more efficient and productive than Winkler sampling for epigaeic ants. Winkler sampling was found to catch greater numbers of smaller ants than pitfall trapping, whereas pitfall trapping caught larger ants. It is these methods of capture and this fundamental understanding of ant anatomy and behavior that I have presented in this paper that will hopefully add to the scientific community’s base of understanding on this subject.

Works Cited

Sagers, C.L., S.M.Ginger, R.D.Evans. “Carbon and Nitrogen isotopes trace nutrient exchange in an ant-plant mutualism,” Oecologia, Vol. 123, 2000, 582-586.

Agrawal, Anurag A. and Benjamin J. Dubin-Thaler. “Induced Responses to herbivory in the NeoTropical ant-plant association between Azteca ants and Cecropia trees: responses of ants to potential inducing clues,” Journal of Behavior, Ecology, and Sociobiology, Vol. 45, 1999, 47-54.

Andrade, J.C. de, and J.P.P. Carauta. The Cecropia- Azteca Association: a case of mutualism? Biotropica, Vol 14:15, 1982.

Bolton, B. Synopsis and classification of Formicidae. Mem. Amer. Entomology Institute. Vol. 71, 1-370, 2003.

Choe, J. C., and D. L. Perlman. 1997. Social conflict and cooperation among founding queens in ants (Hymenoptera: Formicidae). Pp. 392-406 in J. C. Choe and J. Crespi, eds. The evolution of social behavior in insects and arachnids. Cambridge University Press, Cambridge, England.

Janzen, Daniel H. “Alleopathy by Myremecophytes: The ant Azteca as an Allelopathic Agent of Cecropia,” Ecology, Vol.50: 1, 1969, 147-153.

Kircher, John. A Neotropical Companion, Princeton University Press, Princeton, New Jersey, 1997, 130-132.

Longino, John T. “ Geographic Variation and Community Structure in Ant- Plant Mutualism: Azteca and Cecropia,” Biotropica, Vol.21: 2, 1989, 126-132.

Longino, J.T. 1991a. Azteca ants in Cecropia trees: taxonomy, colony structure, and behavior. Pages 271-288 in C. Huxley and D. Cutler, editors. Ant-Plant interactions. Oxford University Press, Oxford, U.K.

Purves, William K., Gordon H. Orians, and H. Craig Hellar. Life: The Science of Biology. Fourth Ed, Sinauer Associates, Sunderland, Massachusetts, 1995, 1013-1014.

Schupp, E. W. Azteca protection of Cecropia: ant occupation benefits juvenile trees. Oecologia, Vol 70, 1986, 379-385.

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